1. Field of the Invention
The invention relates generally to a pressure driven pump for pumping fluid from a wellhead. More particularly, the invention relates to a pumping system having a dogbone pumping element on which equal pressure may be applied for the pump and fill strokes.
2. Background Art
Pumps are used for a variety of tasks in the oil and gas industry. In particular, pumps are often used in subsea applications, such as for operating pressure driven subsea equipment (BOPs, gate valves, and the like), for bringing drilling mud to the surface while drilling, and for bringing produced fluids from a completed well to the surface.
Examples of pumping systems are disclosed in various patents. U.S. Pat. No. 6,325,159 to Mott, et al., discloses a plurality of pumping elements for passing drilling mud from a suction conduit to a discharge conduit. A pump draws hydraulic fluid from a reservoir and discharges pressurized working fluid to hydraulic power chambers of pumping elements, to pump drilling mud. The positions of the valves are determined by control logic in a control module. The timing sequence of filling one power chamber of one pumping element with hydraulic fluid while discharging hydraulic fluid from the power chamber of another pumping element is such that the total mud flow from the pumping elements is relatively free of pulsation. The pumping elements may be diaphragm elements or piston elements.
U.S. Pat. No. 6,102,673 issued to Mott, et al. discloses a subsea positive displacement pump with multiple pump elements, each pump element comprising a pressure vessel divided into two chambers by a separating member and powered by a closed hydraulic system using a subsea variable displacement hydraulic pump. The subsea positive displacement pump includes hydraulically actuated valves to ensure proper valve seating in the presence of, for example, cuttings from the drill bit that are present in mud returns from the wellbore. The hydraulically actuated valves also provide flexibility in valve timing and provides quick valve response in high flow coefficient (Cv) arrangements necessary for high volume pumping (e.g., substantially high flow rates).
U.S. Pat. No. 6,592,334 to Butler discloses a hydraulically driven multiphase pump system for pumping a fluid stream from the surface of a well. The system is intended to eliminate pressure spikes and priming problems of the plunger moving toward the extended position. The hydraulically driven multiphase pump system consists of two vertically disposed plungers. The plungers are hydraulically controlled and actuated to work in alternate directions during a cycle using a closed loop hydraulic system. Each cycle is automatically re-indexed to assure volumetric balance in the circuits. An indexing circuit ensures that each plunger reaches its full extended position prior to the other plunger reaching its preset retracted position. A bias member and an acceleration valve are used to prime the indexing circuit for use in low or variable inlet pressure situations. A power saving circuit is used to transfer energy from the extending plunger to the retracting plunger. Butler, therefore, requires a rather complicated system to minimize pressure spikes and losses.
An issue common to many pumping systems is that the pumping elements require a different flow rate of working fluid for the pump and fill functions. Typically, the pumping elements may be actuated by pressurized working fluid in only one direction, whereas the working fluid must be subsequently drawn out by suction created elsewhere in the system, such as during the pump stroke of another pumping element. This complicates the timing and sequencing of the multiple pumping elements required to produce a relatively uniform flow rate. A related issue is that operating multiple pumping elements may require multiple supply lines if the required fill and pump pressures are different. Yet another issue common to pumping systems is the need to maintain pressure in the system to prevent harmful or even catastrophic separation of various fluid components.